Conductor Ampacity, Termination Temperature, and the Small-Conductor Rule

Sizing a wire is more than reading one number off a table. Three rules work together: the ampacity table (how much a wire can carry), the termination temperature rule (which column you're actually allowed to use), and the small-conductor rule (a hard cap on the breaker for small wires).

1. The ampacity table — Table 310.16

CEC Table 310.16 gives the allowable ampacity of insulated conductors, organized by size and by insulation temperature rating (60°C, 75°C, 90°C). The table's baseline conditions:

• ambient temperature 30°C (86°F), and
• not more than three current-carrying conductors in the raceway or cable.

If your install is hotter or has more than three current-carrying conductors, you must apply correction/adjustment factors — and those start from the 90°C column. That's the main job of the 90°C column: it's the starting point for derating, not a number you terminate on.

2. Termination temperature — 110.14(C)

You can't use a conductor's full 90°C ampacity at a lug that's only rated 60°C or 75°C — the weakest link wins. CEC 110.14(C):